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First tracked space rock recovered after impact

By David Shiga

The discovery of meteorites from an asteroid that exploded over Sudan in October completes an astronomical trifecta. For the first time, scientists have detected a space rock ahead of a collision with Earth, watched it streak through the atmosphere, and then recovered pieces of it.

Analysis of the meteorites could shed light on conditions in the early solar system more than 4 billion years ago.

When the asteroid, called 2008 TC3, was discovered on 6 October last year, it was just 20 hours away from hitting Earth. Though the warning period was short, it was the first time a space rock had been found before it impacted the planet.

Orbital calculations predicted the object would plunge into the atmosphere above Sudan at 0246 GMT on 7 October, and it arrived right on time. Observations suggested it was no more than 5 metres across, too small to survive intact all the way to the ground and cause damage.

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The brilliant fireball it made as it descended through the atmosphere was seen far in the distance by the crew of a KLM airliner, and was observed by various satellites, including a weather satellite called Meteosat-8.

Now, a team of meteorite hunters has found fragments of the object. The meteorites are a unique group in that they come from an object seen hurtling through space before its plunge into Earth’s atmosphere.

Numerous fragments

Students from the University of Khartoum, led by Dr Muawia Shaddad, found the first fragments using data provided by NASA to home in on where fragments were likely to be found.

Scientists involved with the discovery, including Peter Jenniskens of the SETI Institute in Mountain View, California, have reportedly submitted a study about the find to a scientific journal, and have not responded to interview requests.

But Lindley Johnson, head of NASA’s Near-Earth Object Program office at the agency’s headquarters in Washington, DC, reported the find on Monday in Vienna, at a United Nations meeting discussing near-Earth object (NEO) impacts. An image of the first fragment found is included in the slides from Johnson’s presentation (pdf) (see slide 19).

Donald Yeomans, who manages NASA’s efforts to find and track NEOs at the Jet Propulsion Laboratory (JPL) in Pasadena, California, confirmed that “quite a few” fragments have been found but declined to discuss them further.

Weak material?

Before the fragments were found, meteorite expert Peter Brown of the University of Western Ontario in Canada said the asteroid was likely made of relatively weak material, given that 2008 TC3 broke up unusually quickly once it hit the atmosphere, exploding about 37 kilometres above ground.

Another object known to have broken up at about this height scattered fragments over Tagish Lake in Canada in 2000. The Tagish Lake meteorites turned out to be made of a very crumbly material, and fall into a class of meteorites called carbonaceous chondrites, which have been modified little by heat or other processes since the solar system formed more than 4.5 billion years ago.

“I would caution making direct compositional comparisons [with the Tagish Lake meteorites], but it does certainly underscore the global weakness of [2008 TC3],” Brown said in comments posted on the JPL website in November. He added that observations of the rock’s quick breakup “all but rule out” a composition rich in iron.

Point of origin

When the analysis of these rock fragments does come out, what is it likely to tell us? Meteorites in general provide a valuable record of conditions in the early solar system, such as temperature and chemical composition.

And the 2008 TC3 meteorites could be especially illuminating because the parent object was observed in space before the breakup, allowing scientists to calculate its former orbit around the Sun. This provides precious information connecting the meteorites to their place of origin in the solar system.

For most other meteorites, such calculations involve a lot of guesswork. Meteorites had previously been recovered after about 10 “fireball” events, where parent space rocks were observed streaking through the sky.

But in those cases, scientists had to try to reconstruct the object’s orbit based on its path through the atmosphere.

“It’s often very difficult to get from a streak in the sky to what the orbit was,” says Allan Treiman of the Lunar and Planetary Institute in Tucson, Arizona. “But if they’ve got its location before it hit the atmosphere, they’re far better off – that’s really wonderful.”